Method of manufacturing an electrode active material particle for a rechargeable battery

ABSTRACT

A rechargeable battery includes electrodes containing particles of an active material capable of being alloyed with Li. A metallic element that is not alloyed with Li is distributed by its diffusion in particles of the active material.

TECHNICAL FIELD

The present invention relates to a method of manufacturing an electrodeactive material particle for a recharceable battery, and morespecifically to a method of manufacturing an electrode active materialparticle for a rechargeable battery which employs, an active materialcapable of being alloyed with Li.

BACKGROUND ART

Attention has been paid to a lithium rechargeable battery which employsa lithium metal as a negative electrode, as a next generationrechargeable battery because of its high energy density. However, sincethe lithium metal is employed for the negative electrode, the lithiummetal is dissolved and deposited during charge and discharge, therebyproducing dendrites and deforming the electrode. For that reason, thistype of lithium rechargeable battery is inferior in cycle characteristicand cannot be put to practical use. To solve such a problem, proposedare Li-alloy negative electrodes employing a metal capable of beingalloyed with Li as well as carbon negative electrodes employing a carbonmaterial such as graphite. Some of the carbon negative electrodes havebeen put to practical use.

Nevertheless, since the theoretical capacity of the carbon negativeelectrode is as low as 372 mAh/g, the energy density of the carbonnegative electrode is disadvantageously low compared with the negativeelectrode which employs the metal lithium. Furthermore, when theLi-alloy negative electrode is employed, expansion and shrinkage involume are repeated during charge and discharge. Due to this, the activematerial particles are pulverized during charge-discharge cycles,thereby disadvantageously deteriorating the cycle characteristics of thebattery.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a method ofmanufacturing an electrode active material particle for a rechargeablebattery which employs an electrode containing particles of an activematerial capable of being alloyed with Li, which can suppress the activematerial particles from being pulverized and which can surprisinglyimprove the cycle characteristics of the battery.

A method of manufacturing an electrode active material particle for arechargeable battery according to the present invention is characterizedin that a metal element incapable of being alloyed with Li isdistributed by its diffusion into an active material particle.

In the first aspect according to the present invention, a concentrationof the metal element incapable of being alloyed with Li increases froman interior of the active material particle toward a surface of theactive material particle.

In the second aspect according to the present invention, a concentrationof the metal element incapable of being alloyed with Li decreases froman interior of the active material particle toward a surface of theactive material particle.

In the present invention, the active material particle capable of beingalloyed with Li is not limited to a specific one as long as it is madeof a material capable of being alloyed with Li. From a viewpoint ofobtaining high electrode capacity, it is preferable that the activematerial particle is at least one selected from Si, Ge, Sn, Al and In.Si is more preferable since it has a high theoretical capacity.

In the present invention, the metal element incapable of being alloyedwith Li which is diffused into the active material particle is notlimited to a specific one as long as it is a metal element incapable ofbeing alloyed with Li. It is, however, preferable that the metal elementis a material having an excellent conductive property. From thisviewpoint, Cu is particularly preferable.

Accordingly, in a preferred embodiment according to the presentinvention, the rechargeable battery is characterized in that the activematerial particle substantially consists of Si and the metal element isCu.

The active material particle according to the present invention can beused as either a negative electrode active material or a positiveelectrode active material. However, the standard potential of the activematerial capable of being alloyed with Li relative to lithium metal isgenerally low, so that the active material particle of the presentinvention is considered to be used as a negative electrode activematerial in general.

Effect and Advantage

An electrode active material is generally produced by binding a powderyactive material using a binder. Therefore, a location in which electrodereaction occurs is the powder itself as an active material. If Li isabsorbed by the powdery active material, Li is absorbed from the surfaceof the powder first. In general, if Li is absorbed an active materialand alloys with the active material, the volume expands so that thesurface of the powder expands. However, since Li does not enter theinterior of the active material, the interior of the active materialdoes not expand. Therefore, the ratio of expansion of the surfacegreatly differs from that of the interior, with the result that thepowder is pulverized. By adjusting the difference in expansion ratiobetween the surface and the interior, it is possible to suppress theactive material from being pulverized. According to the presentinvention, by distributing the metal element incapable of being alloyedwith Li by its diffusion into the active material particle, thedifference in expansion ratio between the surface and the interior isdecreased to suppress the particle from being pulverized.

In the first aspect of the present invention, provided is aconcentration profile in which the metal element incapable of beingalloyed with Li increases from the interior toward the surface of theactive material particle. In this case, on the surface, theconcentration of the metal element incapable of being alloyed with Li ishigh and the concentration of the active material is low. Due to this,even if Li is absorbed by the active material, the expansion ratio ofthe surface of the active material is low and the difference inexpansion ratio between the surface and the interior in which Li is notabsorbed is not so great. Therefore, an internal stress decreases,making it possible to suppress the active material particle from beingcracked.

In the second aspect of the present invention, provided is aconcentration profile in which the metal element decreases from theinterior toward the surface of the active material particle. In thiscase, the concentration of the active material on the surface of theparticle is high. Therefore, the surface greatly expands and theinterior expands less, thereby increasing an internal stress. However,the concentration of the active material is low in the interior of theparticle, so that the crack formed at the surface of the particle doesnot reach the interior thereof and only the surface of the particlecracks. As a result, the particle is not pulverized as a whole and keptin a state in which the particle is suppressed from being pulverized.

According to the first aspect of the present invention, various methodsmay be considered as methods of manufacturing an active materialparticle in which the concentration of the metal element incapable ofbeing alloyed with Li increases from the interior toward the surface ofthe active material particle. They include, for example, the followingmethods:

(1) A method of providing a layer of a metal element incapable of beingalloyed with Li on the surface of an active material particle capable ofbeing alloyed with Li by electroless plating and then heat treating theactive material particle at an appropriate temperature to therebydiffuse the metal element from the surface into the interior of theactive material particle.

If Si powder is used as the active material particle capable of beingalloyed with Li and Cu is used as the metal element, then a Cu layer isformed on the surface of the Si powder by electroless plating, a heattreatment is then conducted to thereby diffuse Cu into the Si powder andit is possible to provide a concentration profile in which theconcentration of Cu continuously increases from the interior toward thesurface of the Si powder.

(2) A method of providing a layer of a metal element incapable of beingalloyed with Li on the surface of an active material particle capable ofbeing alloyed with Li by a mechano-fusion method and then heat treatingthe active material particle at an appropriate temperature to therebydiffuse the metal element from the surface into the interior of theactive material particle.

If Si powder is used as the active material particle capable of beingalloyed with Li and Cu is used as the metal element, then the Si powderis mechanically mixed with Cu fine particle, a Cu layer is formed on thesurface of the Si powder by the mechano-fusion method, a heat treatmentis then conducted at an appropriate temperature to thereby diffuse Cuinto the Si powder and it is possible to provide a concentration profilein which the concentration of Cu continuously increases from theinterior toward the surface of the Si powder.

According to the second aspect of the present invention, various methodsmay be considered as methods of manufacturing an active materialparticle in which the concentration of the metal element incapable ofbeing alloyed with Li decreases from the interior toward the surface ofthe active material particle capable of being alloyed with Li. Theyinclude, for example, the following methods:

(3) A method of providing a layer of an active material capable of beingalloyed with Li on the surface of a metal particle incapable of beingalloyed with Li by electroless plating and then heat treating the metalparticle at an appropriate temperature.

For example, if Ge is used as the active material capable of beingalloyed with Li and Cu is used as the metal element incapable of beingalloyed with Li, the active material particle can be produced by such amethod.

(4) A method of providing a layer of an active material capable of beingalloyed with Li on the surface of a metal particle incapable of beingalloyed with Li by a mechano-fusion method and heat treating the metalparticle at an appropriate temperature.

If Si is used as the active material capable of being alloyed with Liand Cu is used as the metal element incapable of being alloyed with Li,then Cu powder and Si fine particle are mechanically mixed with eachother, an Si layer is formed on the Cu powder by the mechano-fusionmethod, a heat treatment is conducted at an appropriate temperature tothereby diffuse Si into the Cu powder and it is possible to provide aconcentration profile in which Cu decreases from the interior toward thesurface of the particle.

(5) A method of providing a layer of an oxide of an active material onthe surface of a metal particle incapable of being alloyed with Li by amechano-fusion method or the like, then reducing the oxide at anappropriate temperature in a reducing atmosphere such as hydrogenairflow and also diffusing this reduced active material into the metalparticle.

If Si is used as the active material capable of being alloyed with Liand Cu is used as the metal element incapable of being alloyed with Li,then an SiO layer or an SiO₂ layer is formed on the Cu powder by themechano-fusion method or the like, this oxide is reduced at anappropriate temperature in a reducing atmosphere such as hydrogenairflow and, at the same time, Si formed by reduction is diffused intothe Cu particle and it is possible to provide a concentration profile inwhich the concentration of Cu decreases from the interior of theparticle toward the surface thereof.

In case of methods (1) to (5) stated above, the temperature of the heattreatment conducted to diffuse the metal element is preferably within arange of temperature raised from room temperature by about 1/10 to 4/5of the melting point of the metal to be diffused with reference to theabsolute temperature.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a Cu concentration profile obtained by SIMS analysis aftersubjecting a heat treatment to an Si film formed on a Cu foil.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is below described in more detail by way ofExamples. It will be recognized that the following examples merelyillustrate the particle of the present invention but are not intended tobe limiting thereof. Suitable changes and modifications can be effectedwithout departing from the scope of the present invention.

[Production of Battery of the Present Invention]

[Production of Negative Electrode Active Material]

100 g of Si powder having an average particle diameter of 1 μm wasimmersed into 500 cc of an aqueous solution having a composition asshown in Table 1 at a temperature of 35° C. for 3 minutes, and thenwashed and immersed into 10% by volume of an HCl aqueous solution for 5minutes, thereby forming Pd nuclei serving as a catalyst for electrolessplating on the surface of the Si powder.

TABLE 1 Substance Chemical Formula Concentration Palladium ChloridePdCl₂ · 2H₂O 0.2 g · dm⁻³ Stannous Chloride SnCl₂ · 2H₂0  15 g · dm⁻³Concentrated HCl 200 cm³ · dm⁻³ Hydrochloric Acid

Next, this Si powder was immersed into an electroless plating bath beingadjusted to have a pH of 12.5 and having the composition shown in Table2, until the surface of the Si powder is Cu-colored. The Si powder onthe surface of which a Cu layer was thus formed, was subjected to a heattreatment at 500° C. in vacuum, thereby diffusing Cu into the Si powder.

TABLE 2 Substance Concentration Copper Sulfate 7.5 g · dm⁻³ RochelleSalt  85 g · dm⁻³ Formaldehyde 22 cm³ · dm⁻³ Stabilizer (Methanol) 7.5%

To examine that Cu was diffused into the Si powder as a result of theheat treatment, an Si film having a thickness of 2 μm, instead of the Sipowder, was formed on a Cu foil by the CVD method, and subjected to aheat treatment. It was confirmed that Cu was diffused into the Si thinfilm by SIMS analysis. FIG. 1 shows this result. As shown in FIG. 1, Cuis present in the Si thin film and diffused into the Si thin film. Inaddition, as shown in FIG. 1, Cu increases from the interior of the Sithin film toward the surface thereof. Accordingly, it is understood thatthe Si powder also has Cu diffused thereinto and has a Cu concentrationprofile that the concentration of Cu increases from the interior of theSi powder toward the surface thereof.

(Production of Working Electrode)

100 g of the negative electrode active material produced above was mixedwith an N-methyl-pyrrolidone solution which fluororesin (PVdF), servingas a binder, is dissolved to have a concentration of 5%, the resultantmixture was smashed and mixed with a smash and mixing machine for 30minutes to thereby prepare a slurry. This slurry was applied on anelectrolytic copper foil having a thickness of 18 μm by the doctor bladetechnique, dried and cut into dimensions of 2×2 cm, thereby obtaining aworking electrode.

(Production of Counter Electrode)

An Li metal having a thickness of 0.9 mm was cut into dimensions of 3×3cm, thereby obtaining a counter electrode.

(Production of Test Cell)

The working electrode and the counter electrode produced as stated abovewere superposed on each other through a polypropylene separator, thesuperposed electrodes were put between glass plates and immersed into anelectrolyte, thereby producing a test cell. The Li metal was immersedinto the electrolyte so as not to contact with these electrodes, and wasemployed as a reference electrode. As the electrolyte, a solutionobtained by dissolving 1 mol/litter of LiPF₆ into a mixed solvent ofethylene carbonate and diethyl carbonate in equivalent volume wasemployed.

[Production of Comparative Battery]

A comparative test cell was produced in the same manner as that of theabove-stated test cell except that 100 g of Si powder having an averageparticle diameter of 1 μm was used as a negative electrode activematerial as it is. The obtained cell was referred to as ComparativeBattery A. A comparison test cell was also produced as in the samemanner as that of the above-stated test cell except that 100 g of coppersilicide powder having an average particle diameter of 1 μm was used asa negative electrode active material as it is. The obtained cell wasreferred to as Comparative Battery B.

[Charge-Discharge Cycle Test]

A charge-discharge cycle test was conducted to each test cell. Each cellwas charged to 0V with Li reference, discharged to 2V with Li referenceand a charge-discharge current was 0.5 mA. Table 3 shows the dischargecapacity and charge-discharge efficiency of each test cell.

TABLE 3 Inventive Battery Comparative Comparative Charge Battery ABattery B Dis- Dis- Dis- charge charge charge Charge- Charge- NumberCapac- Effi- Capac- Discharge- Discharge Discharge of ity ciency ityEfficiency Capacity Efficiency Cycles mAh/g % mAh/g % mAh/g % 1 2000 91640 21 610 48 2 1990 99 320 50 450 74 5 1950 98 150 82 350 87 10 1930 97130 82 295 87

As shown in Table 3, Inventive Battery exhibited high discharge capacityand good charge-discharge efficiency even if the number of cyclesincreases, compared with Comparison Batteries A and B. The test cellsafter 10 cycles were decomposed for investigation. In case of battery ofthe present invention, the negative electrode active material in thetest cell was slightly separated from the Cu foil serving as a currentcollector, while the shape of the negative electrode active material waskept. In case of the comparison batteries A and B, by contrast, most ofthe negative electrode active material was separated from the currentcollector, and the shape of the negative electrode active materialitself was not kept. The active material was pulverized and most of thenfell off and were dispersed the electrolyte.

As can be understood from the above, in the battery of the presentinvention, pulverization of the active material is not caused bycharge-discharge cycle test and therefore the battery of the presentinvention exhibits excellent cycle characteristics.

UTILITY IN INDUSTRY

According to the present invention, it is possible to suppress activematerial particles from being pulverized and to greatly improve cyclecharacteristics.

1. A method of manufacturing an electrode active material particle for arechargeable battery, characterized by forming a layer of a metalelement incapable of being alloyed with Li on a surface of the activematerial particle capable of being alloyed with Li and then conducting aheat treatment to diffuse said metal element from the surface into aninterior of the active material particle so that said active materialparticle has a concentration profile in which a concentration of saidmetal element increases from the interior toward the surface of saidactive material particle.
 2. The method of manufacturing an electrodeactive material particle for a rechargeable battery according to claim1, characterized in that the active material particle capable of beingalloyed with Li contains, as an active material, at least one selectedfrom Si, Ge, Sn, Al and In.
 3. The method of manufacturing an electrodeactive material particle for a rechargeable battery according to claim1, characterized in that said metal element is Cu.
 4. The method ofmanufacturing an electrode active material particle for a recharceablebattery according to claim 1, characterized in that the active materialparticle capable at being alloyed with Li substantially consists of Si,and said metal element is Cu.
 5. The method of manufacturing anelectrode active material particle for a rechargeable battery accordingto claim 1, characterized in that the active material particle havingsaid metal element diffused thereinto is used as a negative electrodeactive material.
 6. A method of manufacturing an electrode activematerial particle for a rechargeable battery characterized by forming alayer containing Cu on a surface of the active material particlecontaining at least one selected from Si, Ge, Sn, Al and In and thenconducting a heat treatment to diffuse said Cu into said active materialparticle so that said active material particle has a concentrationprofile in which a concentration of said Cu increases from an interiortoward the surface of said active material particle.
 7. The method ofmanufacturing an electrode active material particle for a rechargeablebattery according to claim 6, characterized in that said active materialparticle is used as a negative electrode active material.